Inter-operability of Swift arrays with C?

Question

How can one pass or copy the data in a C array, such as

float foo[1024];

, between C and Swift functions that use fixed size arrays, such as declared by

let foo = Float[](count: 1024, repeatedValue: 0.0)

?

Solution 1

I don't think this is easily possible. In the same way as you can't use C style arrays for parameters working with a NSArray.

All C arrays in Swift are represented by an UnsafePointer, e.g. UnsafePointer<Float>. Swift doesn't really know that the data are an array. If you want to convert them into a Swift array, you will have create a new object and copy the items there one by one.

let array: Array<Float> = [10.0, 50.0, 40.0]

// I am not sure if alloc(array.count) or alloc(array.count * sizeof(Float))
var cArray: UnsafePointer<Float> = UnsafePointer<Float>.alloc(array.count)
cArray.initializeFrom(array)

cArray.dealloc(array.count)

Edit

Just found a better solution, this could actually avoid copying.

let array: Array<Float> = [10.0, 50.0, 40.0]

// .withUnsafePointerToElements in Swift 2.x
array.withUnsafeBufferPointer() { (cArray: UnsafePointer<Float>) -> () in
    // do something with the C array
}

Solution 2

As of Beta 5, one can just use pass &array The following example passes 2 float arrays to a vDSP C function:

let logLen = 10
let len = Int(pow(2.0, Double(logLen)))
let setup : COpaquePointer = vDSP_create_fftsetup(vDSP_Length(logLen), FFTRadix(kFFTRadix2))

var myRealArray = [Float](count: len, repeatedValue: 0.0)
var myImagArray = [Float](count: len, repeatedValue: 0.0)
var cplxData = DSPSplitComplex(realp: &myRealArray, imagp: &myImagArray)

vDSP_fft_zip(setup, &cplxData, 1, vDSP_Length(logLen),FFTDirection(kFFTDirection_Forward))

Solution 3

The withUnsafePointerToElements() method was removed, now you can use the withUnsafeBufferPointer() instead, and use the baseAddress method in the block to achieve the point

let array: Array<Float> = [10.0, 50.0, 40.0]
array.withUnsafeBufferPointer { (cArray: UnsafePointer<Float>) -> () in
    cArray.baseAddress
}

Comments

[Abhi Beckert]

How about an example that doesn't show real world code? It's not very clear which parts are what you're trying to demonstrate and which are just part of the C API you're using.

[jhabbott]

Surely the array with unsafe pointer must copy the contents, as it doesn't know how long it will stay allocated?

[Ephemera]

@jhabbott That's why it's "unsafe" :). Depending on the internal representation of Array<> it may have to perform a copy to a contiguous memory area if it doesn't already exist in one, but this is probably reasonably unlikely/infrequent in practice.

[marchinram]

Looks like withUnsafePointerToElements was removed: developer.apple.com/library/mac/releasenotes/General/…

[Cameron Lowell Palmer]

@AbhiBeckert DSPSplitComplex is a part of the Apple Accelerate framework which is a C API and is a good example. It is a simple struct for holding a vector of complex numbers.

[Nicolas Buquet]

As of Swift 3, Array init is now: var myRealArray = [Float](repeating: 0.0, count: len)

[tatiana_c]

U saved my life